Characterization and Quantification of Offshore Methane Emissions using Airborne in situ Measurements of Methane, Ethane and Methane Isotopologues

Abstract

Methane (CH4) is the second-most important long-lived anthropogenic greenhouse gas after carbon dioxide and, because of its short lifetime, is an attractive target for rapid emission reduction. Atmospheric CH4 mole fractions have almost tripled since preindustrial times due to human activity. A concurrent decline in the 13CH4/12CH4 isotopic ratio of CH4 (expressed as δ13C(CH4)) since 2007 points to a significant change in CH4 sources and sinks, since those vary in their δ13C(CH4) signatures. So far, the understanding of the underlaying drivers is insufficient, which hampers both prioritizing mitigation actions and predicting future CH4 trends. This dissertation aims at a more detailed characterization of anthropogenic CH4 emissions using airborne in situ measurement methods. Thereby, the focus is on the sector of offshore fossil fuel production, which is understudied so far. Driven by the hypothesis that emission inventories using generic scaling methods are not able to estimate emissions with sufficient accuracy, airborne measurement data gathered around offshore installations is analyzed and compared with different inventory data. Moreover, it is investigated, whether emissions can be better characterized by using the latest airborne laser spectroscopy methods. To this end, an existing direct laser absorption spectrometer is adapted for the high-resolution and continuous airborne measurement of the tracers C2H6 and δ13C(CH4), and deployed on research aircraft to study offshore emissions. The first part presents the analyses of an airborne study conducted by the British Antarctic Service (BAS) in the southern North Sea in 2019. CH4 emission rates from offshore gas installations were derived by applying the well-established mass balance method. They were then compared with direct operator-reporting, estimates from regional point source inventories and a globally gridded inventory, which uses national reported emissions in the framework of the United Nations Framework Convention on Climate Change (UNFCCC) and infrastructure data for a spatial downscaling. The findings reveal significant deviations between the derived emission rates and the estimates in the existing inventories. The inventories underestimate emissions by factors from 6 to 279, with the global inventory deviating the most. Notably, the operator-based facility-level reporting corresponds with the calculated flux, which only deviates by a factor of 0.64. The comparison with estimates from airborne measurements in other offshore regions shows that CH4 emission rates are comparable and do not depend on oil- and gasproduction rates, which are typically used for emission estimates in inventories. The second part of the thesis describes the characterization and adaptation of the DLR-QCLS, a fast and precise Quantum-cascade and Interband-cascade laser spectrometer, enabling it to measure the tracers C2H6 and isotopic CH4 (13C(CH4); 12C(CH4)), to then use the isotopic ratio δ13C(CH4) to derive source signatures from offshore point sources. The laboratory characterization shows that, given the instrument precision (0.86‰ (1σ, 2 Hz)), source detection is feasible for strong fossil fuel plumes larger than approximately 250 ppb CH4, depending on the individual source signature. Furthermore, the qualitative detection of source signatures is demonstrated. During two aircraft campaigns the DLR-QCLS detected C2H6 and δ13C(CH4) signals from offshore oil installations, from which characteristic C2H6 to CH4 ratios and source signatures were calculated. Nonetheless, it is necessary to refine the methodology both in laboratory settings and during field operations in order to achieve a better correspondence between the quantitative measurement of δ13C(CH4) and estimated source signatures.